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Polyurethane Hybrid Synthesis in the Presence of Water

SpecialChem / Andy Extance – May 6, 2009

Waterborne polyurethanes combine good adhesion characteristics with inherently low emissions of volatile organic compounds. Hybrid systems are increasingly being used, in which a number of reaction stages, including radical polymerisation, occur in water. However many steps are required to get to this stage, and all of them potentially have an impact on the final properties of the adhesive. At the simplest level, polyurethanes are usually produced by a reaction between polyisocyanate and polyol molecules. The highly reactive, and hence toxic, isocyanates can react rapidly with water to form symmetrical ureas. With polyisocyanates this can constitute a kind of polymerization, but it would not give the chain lengths associated gained in the intended reaction with polyols. Consequently polyurethane polymerization must be done under water-free conditions. Similarly, adding another chemical group to the isocyanate at the end of the chain avoids self-condensation on the introduction of water, which helps to keep the mixture's viscosity down.

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